This invention relates generally to ethylene polymerization, and more specifically to methods and apparatus for use of chromium-based catalysts for the production of polyethylene in a gas phase polymerization reactor, and in particular, chromium based catalysts which are chromium oxide based.
Polyethylene resins used in blow molding and extrusion applications require stiffness, impact resistance, and environmental stress crack resistance (ESCR). In processing, these materials need to be able to run at high rates without process instabilities, unacceptable swell, or excessive head pressure or temperature. The majority of polyethylene products used in these applications are produced with chromium catalyst systems in slurry or gas-phase processes. In designing a polyethylene product for these applications, there is a trade-off between properties. ESCR can be increased by decreasing density (crystallinity) and/or increasing molecular weight (or high molecular weight component) and/or preferentially placing short chain branches (SCB) on the higher molecular weight molecules. A decrease in density will increase the ESCR while sacrificing some stiffness. Present chromium oxide catalyst technology can produce products with a balance of properties essentially equal to industry standard products. Silyl chromium (also called “silyl chromate” herein) catalyzed products offer superior physical properties (ESCR, impact) at a given density, however, because of a greater high molecular weight tail, these silyl chromium-produced products exhibit higher swell and higher head pressure in blow molding processing.